The present invention relates generally to lasers and more particularly to a semiconductor vertical-external-cavity surface-emitting laser that generates second harmonic light by means of a nonlinear crystal contained within an optical cavity of the laser.
Compact efficient sources of coherent blue or green laser light at wavelengths in the range of 400-550 nanometers are useful for many different types of applications including high-density optical data storage and retrieval, laser printing, optical image projection, fluorescence-based chemical-sensing, materials processing and optical metrology. Many different semiconductor laser approaches have been explored to generate lasing in this wavelength range, but with limited success and reliability. Prior approaches using external frequency doubling of semiconductor lasers have generated only a few nanoWatts of blue lasing. What is needed is a compact laser source which can generate light in the wavelength range of 400-550 nanometers at milliWatt output powers and with good beam quality.
The present invention overcomes the limitations of the prior art by providing a frequency-doubled vertical-external-cavity surface-emitting laser (VECSEL) which generates up to 5 milliWatts or more of lasing output at a wavelength below about 500 nanometers.
An advantage of the present invention is that the frequency-doubled VECSEL operates in a fundamental transverse mode to provide excellent focusability and beam propagation.
Yet another advantage of the present invention is that embodiments of the frequency-doubled VECSEL of the present invention can be activated either optically with an external laser pump energy source, or electrically by using current injection from an external power supply.
A further advantage of the present invention is that a precise wavelength control over a fundamental lasing frequency in the VECSEL can be provided during fabrication for precisely matching the fundamental frequency to be within an acceptance bandwidth of a particular nonlinear crystal to permit efficient frequency doubling and the generation of light at a predetermined wavelength.
Still another advantage of the present invention is that an air gap is provided between a gain element and a nonlinear crystal in the VECSEL; and this air gap permits the insertion of an optional Fabry-Perot etalon for reducing a bandwidth for lasing within the device, thereby improving the coherence and stability of the frequency-doubled lasing output.
These and other advantages of the present invention will become evident to those skilled in the art.
The present invention relates to an apparatus for generating light at a second-harmonic frequency. The apparatus comprises a semiconductor substrate which includes a first reflector formed on the substrate and a semiconductor active region formed on the substrate proximate to the first reflector; and a nonlinear crystal (e.g. potassium niobate) located proximate to the active region and spaced from the active region by an air gap (e.g. about 1-3 mm), with the nonlinear crystal having a second reflector on a surface thereof away from the active region. The first and second reflectors together define a laser cavity which contains the active region and the nonlinear crystal, with the active region generating lasing light at a fundamental frequency in response to electrical or optical activation, and with the nonlinear crystal converting a portion of the lasing light into light at the second-harmonic frequency. The second-harmonic lasing is emitted from the apparatus through the second reflector which is partially transmissive at the second-harmonic frequency.
The active region preferably comprises a plurality of quantum wells which can be gallium arsenide (GaAs) quantum wells, indium gallium arsenide (InGaAs) quantum wells, or aluminum gallium arsenide (AlGaAs) quantum wells depending upon the fundamental frequency which can be selected to correspond to a wavelength in the range of 600-1100 nanometers to generate light at a second-harmonic frequency that is at a wavelength equal to one-half the wavelength of the lasing at the fundamental frequency.
The first reflector is preferably a Distributed Bragg Reflector formed from a plurality of alternating high-refractive-index and low-refractive-index semiconductor layers epitaxially grown on the substrate to provide a reflectivity for light at the fundamental frequency of xe2x89xa799%. An optional third reflector can be epitaxially grown on the substrate above the active region, with the third reflector preferably being a Distributed Bragg Reflector formed from a plurality of alternating low-refractive-index and high-refractive-index semiconductor layers. The third reflector can help to control and stabilize the fundamental frequency for efficient second-harmonic light generation. An optional Fabry-Perot etalon can also be located within the air gap to narrow a bandwidth of the fundamental frequency for more efficient second-harmonic light generation and to provide a reduced bandwidth and increased coherence for the second-harmonic light.
The apparatus can be optically activated by pump light from a separate pump laser (e.g. a semiconductor laser or a titanium-sapphire laser). Alternately, the apparatus can be electrically activated by including a semiconductor p-n or p-i-n junction within the active region. For an electrically-activated device, an upper electrode can be provided above the active region, and a lower electrode can be provided on the substrate.
The present invention is further related to a semiconductor laser which comprises a gallium arsenide substrate having a plurality of III-V compound semiconductor layers epitaxially grown thereon, including a plurality of alternating high-refractive-index and low-refractive-index semiconductor layers forming a first reflector which is reflective of light at a fundamental frequency, and an active region wherein light is generated at the fundamental frequency; and a nonlinear crystal (e.g. potassium niobate) separated from the substrate and plurality of semiconductor layers by an air gap (e.g. a 1-3 mm air gap), with the nonlinear crystal having a first surface and a second surface, the first surface nearest the substrate generally being substantially planar and including an anti-reflection coating thereon, and the second surface preferably being curved (e.g. a 15-millimeter radius of curvature) and including a second reflector which is reflective of the light at the fundamental frequency and transmissive of light at a second-harmonic frequency that is twice the fundamental frequency. The first and second reflectors define therebetween a laser cavity that extends from the first reflector through the active region and the nonlinear crystal to the second reflector. The semiconductor laser of the present invention is responsive to an external energy source (e.g. optical or electrical activation) to generate light in the laser cavity at the fundamental frequency, with the nonlinear crystal being operative to convert the light at the fundamental frequency to light at the second-harmonic frequency, so that an output light beam (i.e. a lasing beam) is generated at the second-harmonic frequency and transmitted through the second reflector.
The first reflector, which preferably has a reflectivity for light at the fundamental frequency of  greater than 99%, can comprise, for example, gallium arsenide (GaAs) high-refractive-index layers and aluminum arsenide (AlAs) low-refractive-index layers. The second reflector is also preferably highly reflective at the fundamental frequency (e.g. xe2x89xa799%), while being transmissive of the light generated at the second-harmonic frequency.
To generate blue light at a wavelength of about 490 nanometers corresponding to the second-harmonic frequency, the active region can comprise a plurality of alternating layers of tensile-strained gallium arsenide phosphide (GaAsP), compressively-strained indium gallium arsenide (InGaAs), and aluminum gallium arsenide (AlGaAs) to generate light at a fundamental wavelength of about 980 nanometers. In this preferred embodiment of the present invention, the GaAsP layers can have a semiconductor alloy composition GaAs0.8P0.2, the InGaAs layers can have the semiconductor alloy composition In0.18Ga0.82As, and the AlGaAs layers can have the semiconductor alloy composition Al0.08Ga0.92As. For this embodiment of the present invention, the active region can further include a carrier-confinement layer comprising aluminum gallium arsenide (AlGaAs), and an indium gallium phosphide (InGaP) cap layer overlying the AlGaAs current-confinement layer. In other embodiments of the present invention, the active region can comprise a plurality of alternating layers of gallium arsenide (GaAs) and aluminum gallium arsenide (AlGaAs) to generate a fundamental wavelength of about 860 nanometers which can be converted by the nonlinear crystal into blue light at a wavelength of about 430 nanometers. Other embodiments of the present invention can be formed with the semiconductor laser operating at a fundamental frequency corresponding to a wavelength in the range of 600-1100 nanometers for generation of a second-harmonic frequency corresponding to a wavelength in the range of 300-550 nanometers, with the second-harmonic wavelength being equal to one-half the fundamental wavelength.
Embodiments of the present invention can be provided for either optical activation or electrical activation. For optical activation, the external energy source can comprise optical pumping with lasing light from at least one pump laser, with the lasing light from each pump laser preferably being incident on the plurality of semiconductor layers at a Brewster angle which is defined by the refractive index of the active region. The Brewster angle can be, for example, about 74xc2x0 as measured from an axis normal to an upper surface of the substrate. Each pump laser can comprise, for example, a semiconductor laser or a solid-state laser (e.g. a titanium-sapphire laser).
For electrical activation, the external energy source comprises electrical excitation with an electrical current flowing through a semiconductor p-n or p-i-n junction which can be formed about the active region by doping the plurality of semiconductor layers. An electrically-activated semiconductor laser also preferably includes an upper electrode above the active region and a lower electrode on the substrate. An electrically-activated semiconductor laser can also include an optional third reflector which can be epitaxially grown above the active region as a plurality of alternating low-refractive-index and high-refractive-index semiconductor layers that are reflective of the fundamental frequency. This third reflector can be used for wavelength control of the fundamental frequency for more efficient conversion of light at the fundamental frequency into light at the second-harmonic frequency.
An optional Fabry-Perot etalon can be located within the air gap of either an optically-activated device or an electrically-activated device to narrow the bandwidth of the fundamental frequency in the semiconductor laser.
Additional advantages and novel features of the invention will become apparent to those skilled in the art upon examination of the following detailed description thereof when considered in conjunction with the accompanying drawings. The advantages of the invention can be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.